The present invention relates to a method and apparatus for analyzing the characteristics of substrate surfaces.
In the manufacturing process of an electronic package, many operations require a cleaning step. However, even the most accurate cleaning method can leave some contamination. Many circumstances require that the cleanliness of a surface be tested and measured to verify that the contamination of the part is contained within acceptable limits. Furthermore, a check on the cleanliness of a surface may be required when a part is moved between processing steps where handling or transportation is required. For example a cleaning step or a check of the cleanliness degree may be required before electroplating a part, or before a component is soldered onto a plated pad. An example is the mounting of a chip (device) on a substrate, usually done through soldering: this is called xe2x80x9cfirst level packagingxe2x80x9d. This stage of the process needs to be performed in a xe2x80x9ccleanxe2x80x9d environment to avoid contamination of the parts, before the module is encapsulated, usually with a resin, and the circuits are protected by external agents.
The meaning of xe2x80x9ccleanxe2x80x9d can vary significantly according to the application field considered. In precision cleaning applications, in which the required degree of cleanliness is very high, such as in the manufacture of medical components or electronic products, an accurate assessment of surface cleanliness is needed in order to meet the expected quality parameters. An accurate cleanliness assessment also helps in protecting the environment against pollution from cleaning agents, by evaluating new cleaning methods or optimizing the existing cleaning processes.
In the manufacturing of electronic components and products, the contamination of a surface can be caused by a number of different factors. Examples of contamination are: deposition of small particles; ionic contamination; deposition of chemical compound layers (e.g., oils or salts) occurring during manufacturing steps; adsorption of organic material (e.g., hydrocarbons or moisture) caused by exposure to the atmosphere.
Methods for measuring the degree of cleanliness can be divided into two categories: a) direct methods; and, b) indirect methods. Direct methods analyze the surface to be checked to discover whether the contamination of the surface exceeds a predetermined threshold. These methods either are dependent on human discretion, such as in a magnified visual inspection, or require sophisticated and very expensive equipment. Indirect methods are based on the analysis of a very powerful solvent after it has been used to extract the contaminants from a specimen surface.
One direct method is the so called xe2x80x9cangle of contactxe2x80x9d method. The angle of contact method is based on the measurement of the surface wettability. The contact angle is the angle between the substrate surface and the tangent of a liquid droplet deposited on the substrate, at the point of contact of the liquid droplet with the substrate surface. This contact angle depends on the surface wettability. An ideal, perfect wettability would cause the droplet to spread out over the substrate, giving a contact angle approaching 0xc2x0. A good wettability value would indicate a clean surface, while a bad wettability value is a symptom of contamination of the surface. For example, with inorganic surfaces, the surface wettability is affected by surface organic contamination. By detecting any displacement on the wettability value from the expected one, an accurate estimate of the degree of cleanliness can be derived.
It is known to measure this contact angle by projecting a profile image of the deposited droplet on a screen and estimating the contact angle by measurement on the projected image. One problem with this method is that the measurement of the angle is subject to significant errors and it is unreliable because of human intervention.
U.S. Pat. No. 4,050,822 describes a method for determining the wettability of a material by measuring the height of a drop of a liquid deposited on its surface and comparing the height measurements to standard drop measurements. This method is subject to visual human error in coincident comparison of a drop with a graduated scale and requires an elaborate optical system.
U.S. Pat. No. 5,268,733 describes a method for determining the contact angle by measuring, on a projected image of the droplet, the angle between a base line defined by the substrate surface and a reference line, which connects the contact point of the droplet with the substrate and an apex point of the droplet. This method provides a more reliable measurement of the contact angle, but still suffers from the difficulty of obtaining the measurement and of the human intervention in the determination of the angle.
Another drawback of the xe2x80x9cangle of contactxe2x80x9d method is that it is applicable only to flat, smooth surfaces, otherwise, irregularities of the surface would affect the tangent angle measurement.
For the above reasons a more accurate and reliable method would be highly desirable for the detection of contamination (e.g., organic) on a substrate (e.g., inorganic) surface. It is an object of the present invention to provide a technique which overcomes the above drawbacks.
An object of the present invention is to provide a method for measuring the degree of cleanliness of a substrate surface.
A more specific object of the present invention is to provide such a method adapted for measuring organic contamination on the surface of an inorganic substrate.
Yet another object of the present invention is to provide an apparatus to measure the degree of cleanliness of a substrate surface by depositing a droplet of liquid on a substrate surface and determining the rate of evaporation of the droplet of liquid.
A further object of the present invention is to provide a method to measure the degree of cleanliness of a substrate surface by providing a ratio of the evaporation rate of a droplet of liquid on a substrate surface, and the expected evaporation rate of a drop of liquid on a clean substrate surface.
According to one aspect of the present invention, there is provided a method for analyzing the characteristics of a substrate surface, comprising the steps of depositing a droplet of liquid on a substrate surface, and determining a measure indicative of the evaporation rate of the droplet of liquid, which is indicative of the liquid surface area of the droplet of liquid.
According to another aspect of the invention, there is provided an apparatus for revealing contamination and measuring the cleanliness of a substrate surface comprising a support structure for holding a substrate having a surface, a depositing member for depositing a liquid droplet of a predetermined volume on the surface of the substrate, a measuring structure for measuring the time Te taken by the liquid droplet to lose a predetermined percentage of its weight, and a comparing structure for comparing the measured time Te with an expected value for a clean substrate surface.